Can you ignore SEP 11 clients in SEPM 12?

We're in the process of upgrading from SEP 11 to SEP 12. One of the issues we currently have is that there are a large amount of SEP 11 clients currently checking into our manager that are not actually managed by our IT department. Due to policy changes we're planning on not upgrading these machines when we go to SEP 12 and no longer supporting them under our SEP Manager. The question is if we don't upgrade them will they check into the SEPM once its upgraded from 11 to 12? And if so, is there a way to block them from being able too? Basically disable backwards compatibility within SEPM 12 to manage SEP 11 clients?